Frequent tragedies in airplane transportation have caused concern over the ability of airlines to evaluate the airworthiness of airplanes within their respective fleets. As airframes age, characteristics of materials, which make up airframe components, change due to stresses and strains associated with flights and landings. Moreover, there is a risk that a state of the airframe material may go beyond the point of elasticity (i.e., the point the material returns to its original condition) and extend into the point of plasticizing or worse, beyond plasticizing to failure. As a result, periodic inspections and testing are conducted on airplane components during the airplane component's life cycle. Such inspections and testing are mandated by governing bodies and are largely based on empirical evidence.
Inspections and testing of airplanes are bifurcated into two areas: destructive testing and nondestructive inspection (NDI), nondestructive testing (NDT) or nondestructive evaluation (NDE). “NDI,” as this term is used hereinafter in the specification, encompasses the meanings conveyed by NDT and NDE, as those are described above. The area of destructive testing, as the name implies, requires the airplane component under scrutiny to be destroyed in order to determine the quality of that airplane component. This can result in a costly endeavor because an airplane component that may have passed the procedure is destroyed, and is no longer available for use. Frequently, where destructive testing is done on samples (e.g. coupons) and not on actual components, the destructive test may or may not be reflective of the forces that the actual component could or would withstand within the flight envelope of the airplane.
On the other hand, NDI has the obvious advantage of being directly applied to actual airplane components or sub-components in their actual environment. Several important methods of NDI that are performed in a laboratory setting are listed and summarized below.
Radiography involves inspection of a material by subjecting it to penetrating irradiation. Although effective damage detection has been done using neutron radiation, X-rays are the most familiar type of radiation used in this technique. Most materials used in airplane component manufacturing are readily acceptable to X-rays. In some instances, an opaque penetrant is needed to detect defects.
Real-time X-rays, which are frequently used as part of recent inspection techniques, permit viewing the area of scrutiny while doing a repair procedure. Some improvement in resolution has been achieved by using a stereovision technique where the X-rays are emitted from dual devices which are offset by about 15 degrees. When viewed together, these dual images give a three-dimensional view of the material. Still, the accuracy of X-rays is generally no better than plus or minus 10% void content. Neutrons (N-ray), however, can detect void contents in the plus or minus 1% range. The difficulty in implementing radiography raises safety concerns because a radiation source is being used. Nevertheless, in addition to detecting internal flaws in metals and composite structures using conventional non-radiography related methods, X-rays and neutrons are useful in detecting misalignment of honeycomb cores after curing, blown cores due to moisture intrusion, and corrosion.
Ultrasonic is the most common non-destructive inspection method for detecting flaws in composite materials. The method is performed by scanning the material with ultrasonic energy while monitoring the reflected energy for attenuation (diminishing) of the signal. The detection of the flaws is somewhat frequency-dependent and the frequency range and scanning method most often employed is called “C-scan.” In this method, water is used as a coupling agent between the sending device and the sample. Therefore, the sample is either immersed in water or water is sprayed between the signal transmitter and the sample. This method is effective in detecting defects even in samples that are substantially thick, and may be used to provide a thickness profile. C-scan accuracies can be in the plus or minus 1% range for void content. A slightly modified method call L-scan can detect stiffness of the sample by using the wave speed, but requires that the sample density be known.
Acousto-ultrasonic, another non-destructive inspection method, is similar to ultrasound except that separate sensors are used to send the signal and other sensors are used to receive the signal. Both sensors are, however, located on the same side of the sample so a reflected signal is detected. This method is more quantitative and portable than standard ultrasound.
Acoustic emission, a yet another non-destructive inspection method, involves detecting sounds emitted by a sample that is subjected to stress. The stress can be mechanical, but need not be. In actual practice, in fact, thermal stresses are the most commonly employed. Quantitative interpretation is not yet possible except for well-documented and simple shapes (such as cylindrical pressure vessels).
Thermography (sometimes referred to as “IR thermography”) is yet another non-destructive inspection method that detects differences in relative temperatures on the surface undergoing inspection. Differences in relative temperatures on the inspected surface are produced due to the presence of internal flaws. As a result, thermography is capable of identifying the location of those flaws. If the internal flaws are small or far removed from the surface, however, they may not be detected. In thermography, there are generally two modes of operation, i.e., an active and a passive mode of operation. In the active mode of operation, a sample is subjected to stress (usually mechanical and often vibrational) and the emitted heat is detected. In the passive mode of operation, the sample is externally heated and the resulting thermal gradients are detected.
Optical holography, a yet another non-destructive inspection method, uses laser photography to give three-dimensional pictures, which are called “holography.” This method detects flaws in samples by employing a double-image method, according to which two pictures are taken while stress is induced on a sample between the times when a picture is taken. This method has had limited acceptance because of the need to isolate the camera and the sample from vibrations. However, it is believed that phase locking may eliminate this problem. The stresses that are imposed on the sample are usually thermal. If a microwave source of stress is used, moisture content of the sample can be detected. For composite material, this method is especially useful for detecting debonds in thick honeycomb and foam sandwich constructions. A related method is called shearography. In this method, a laser is used with the same double exposure technique as in holography where stress is applied between exposures. However, in this case an image-shearing camera is used in which signals from the two images are superimposed to provide an interference pattern and thereby reveal the strains in the samples. According to this method, strains are detected in a particular area, and the size of the pattern can give an indication of the stresses concentrated in that area. As a result, shearography allows a quantitative appraisal of the severity of the defect. The attribute of quantitative appraisal, relatively greater mobility of shearography over holography, and the ability to stress the sample using mechanical, thermal, and other techniques, has given this method wide acceptance since its introduction.
Unfortunately, current commercial industry inspection and repair methods suffer from several drawbacks. By way of example, the above described non-destructive inspection methods are largely limited to laboratory analysis. The current commercial industry inspection and repair methods are inefficient, costly and not standardized. As another example, these inspection and repair methods have seen little or no changes in the past 20 or 30 years and have not solved the “Aging Airplane” safety problems. As it stands now, inspection of airplane components are limited to the “Tap Test,” visual inspection, and Eddy Current analysis. Furthermore, inspection timetables are developed and updated primarily as a function of anecdotal evidence, all too frequently based on airline catastrophes.
Despite a wealth of diagnostic tools mostly available in laboratory settings for detecting defects, what is, therefore, needed are novel systems and methods for effective airplane fleet management and that do not suffer from the above-described drawbacks encountered by the current airplane inspection methods and systems.